Why does it dip?
Because the magnetic field of the earth is mostly vertical, it goes out of one pole and into the other. For typical northern city the vertical component is about 4-5x larger than the horizontal, which is a real pain if you are trying to design a compass that can be used when tilted.
- that's why ships compasses are gimbaled to always float level when the ships tips

Why you would own one?
Mainly as an instrument to investigate/demonstrate the earth's field. You could use it to measure your latitude -although a sextant is easier.